Hello NAVE crew! In this video lesson from the web series Vet Anesthesia Unravelled, we gonna talk about Local Anesthetics Pharmacology.
Since the late 19th century, local anaesthetics had been used when cocaine was first used in ophthalmic surgery. However, due to its psychotropic and cardiovascular side effects, the use of cocaine was discontinued thereon. The quality of local anaesthesia changed after lidocaine, in 1948. It was the first amide-derived local anaesthetic, taken molecule stability and non-production of para-aminobenzoic acid (PABA), thus being less allergenic.
*Mechanism of action*
Local anaesthetics produce reversibly block nerve conduction by sodium channels blockade in afferent fibres. The molecular form of the local anaesthetic passes through the phospholipid layer and reaches the inner portion of the cell. After that, this molecule receives an electron, becoming to a protonated form. This form can block the ion channel, preventing cell repolarization.
*Lock sequence*
Blocking speed is dependent on fibre anatomy. The first fibres to be blocked are the C (fine and unmyelinated) and Adelta (fine and myelinated) fibres. Then there is the blockage of the Abeta and Aalpha fibres, which are thicker and myelinated. Thus, the increasing order of blockade is sequenced, with the loss of sensation of pain, heat, touch, deep pressure and, finally, the motor sensation. The return of sensitivity follows the reverse sequence.
*Main anaesthetics*
- Lidocaine: It is still the main local anaesthetic used in veterinary medicine. It is used for locoregional blocks, antiarrhythmic and analgesic, as a continuous intravenous infusion.
- Bupivacaine: It is a long-lasting local anaesthetic, providing anaesthesia for more than 6 hours. However, it also has a prolonged latency period, about 20 minutes, and it is greater toxicity than lidocaine. An interesting feature of bupivacaine is that it blocks sensory fibres, C and Adelta, but not all Abeta fibres. So, the patient maintains some motor autonomy, which can be interesting in some cases.
- Ropivacaine: It "looks like" bupivacaine in terms of latency, action and blocking characteristics. However, it is a little safer in terms of toxic effects.
Some factors can change the blockade effectiveness, such as the volume injected, lipid solubility, concentration and the use with vasoconstrictor. In this case, it is worth mentioning the concentration, which the higher, the longer the anaesthesia will be, and the use of the vasoconstrictor, which increases the period of action by approximately 50%. However, it is important to note that using local anaesthetic WITH vasoconstrictor in the extremities can cause necrosis. So, don't do it.
*Toxicity*
Although local anaesthesia is considered one of the safest anaesthetic modalities, there are some possibilities for complications. Possible neural injuries by a puncture can occur, which can be avoided using guided techniques, CNS toxicity, promoting depression, ataxia, anxiety, seizures, unconsciousness, coma and even bulbar shock, and cardiovascular, promoting myocardial depression.
Find out more watching the video!
👍 LIKE THIS VIDEO? Then check out other videos related to this topic:
✅ Epidural Anesthesia bit.ly/3ytrVxX
✅ Local Block Anesthesia in Ruminants bit.ly/3yCaStE
✅ Local Block Anesthesia in Horses bit.ly/3ucVfoT
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Additional Material
- Bruchim et al. J Vet Emerg Crit Care, 22:419-427, 2012.
- Kapur et al. Acta Anaesthesiol Scand, 51:101-107, 2007.
- Nobrega Neto et al. BMC Vet Res, 9:199, 2013.
- Rezende et al. AJVR, 72:446-451, 2011.
Additional credits
Introduction - Rodrigo Leitão • TACACÁ ///Comida de ru...
Intro music - Baila Mi Cumbia - Jimmy Fontanez
Final music - Level Up - Quincas Moreira
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